The present invention generally relates to satellite broadcast communication systems, and more particularly to a satellite broadcast communication system which broadcasts information from a main station to a plurality of sub stations via a communication satellite and successively transmits the information while receiving reception acknowledge signals from each of the sub stations.
In the satellite broadcast communication system of the type described above, the going line from the main station to the sub station is generally a line of a continuous wave and the returning line from the sub station to the main station is generally a time-division multi access (TDMA) line or a random access line.
Because a large number of sub stations are set up over a large region, it is desirable that the sub station is easy to set up and has a small scale so that the cost of the system can be minimized.
Furthermore, in order to cope with the different weather conditions depending on the locations of the scattered sub stations and to avoid deterioration of the receiving condition at the main station due to the transmission power which is reduced to make the sub station small-scale, it is necessary for each sub station to transmit the reception acknowledge signal a plurality of times. Hence, it is desirable to minimize the time required for the main station to receive the reception acknowledge signal from the sub station.
In the conventional satellite broadcast communication system, one of the following two methods is employed.
According to a first method, a number of time slots equal to the number of sub stations are provided for the returning line from the sub station to the main station, and one time slot is fixedly assigned to each sub station so that the main station can positively receive the reception acknowledge signal from each sub station. In this case, each sub station returns the reception acknowledge signal using the time slot which is assigned thereto, and the main station selects only the desired reception acknowledge signal.
However, according to this first method, even if the broadcast communication is to be made with respect to a specific group of sub stations less than the total number of sub stations, each sub station belonging to this specific group can only make access to the time slot assigned thereto. As a result, the maximum time required for the main station to receive the reception acknowledge signal is (total number of sub stations).times.(length of time slot), and there is a problem in that the time required for the main station to receive the reception acknowledge signal is long. In calculating the maximum time, the satellite delay time is omitted because it is common for all sub stations.
On the other hand, a second method makes the number of time slots of the returning line smaller than the number of sub stations belonging to a group. A random-number generator is provided in each sub station, and a time slot corresponding to the generated random number is used to transmit the reception acknowledge signal to the main station via the communication satellite.
But according to the second method, if the number of sub stations belonging to the group is large or the broadcast communication is to be made with respect to all of the sub stations, the collision rate of the reception acknowledge signals transmitted from the sub stations increases at the communication satellite. In order to avoid the high collision rate, the range of the random number which determines the time slot of each sub station must be enlarged. As a result, there is a problem in that the time required for the main station to receive the reception acknowledge signal increases if the range of the random number is increased.